Adaptive genetic differentiation in a predominantly self-pollinating species analyzed by transplanting into natural environment, crossbreeding and Q _ST-F _ST test

• Both genetic drift and natural selection result in genetic/phenotypic differentiation over space. I analyzed the role of local adaptation in the genetic differentiation of populations of the annual grass Hordeum spontaneum sampled along an aridity gradient. • The study included the introduction of plants having desert vs nondesert origin into natural (desert) environment, analysis of population differentiation in allozymes and random amplified polymorphic DNA (RAPD) markers vs phenotypic traits (Q _ST-F _ST comparison), and planting interpopulation hybrids under simulated desert conditions in a glasshouse. • The results of the home advantage test, Q _ST-F _ST comparison and crossbreeding were consistent with local adaptation; that is, that differentiation of the desert plants from plants of nondesert origin in phenotypic traits was adaptive, giving them home advantage. Each method used provided additional, otherwise unavailable, information, meaning that they should be viewed as complementary rather than alternative approaches. • Gene flow from adjacent populations (i.e. populations experiencing the desert environment) via seeds (but not pollen) had a positive effect on fitness by enhancing natural selection and counteracting drift. At the same time, the effect of genes from the species distributional core (nondesert plants) by either seed or pollen had a negative fitness effect despite its enriching effect on neutral diversity. The pattern of outbreeding depression observed in interpopulation hybrids (F ₁) and their segregating progeny (F ₂) was inconsistent with underdominance, but indicated the presence of additive, dominance and epistatic effects.